1. Field of the Invention
The present invention relates to an image forming apparatus and a method of controlling an image forming apparatus.
2. Description of the Related Art
As an image forming apparatus based on an electrophotography process that outputs a color image, an apparatus having a schematic arrangement shown in FIG. 1 is known. Referring to FIG. 1, reference numerals 1a to 1d denote photosensitive members as image carriers; 2a to 2d, chargers; 3a to 3d, exposure units; and 4a to 4d, developers. Reference numerals 53a to 53d denote primary transfer units; 6a to 6d, cleaners; 51, an intermediate transfer belt; 55, an intermediate transfer belt cleaner; and 56 and 57, secondary transfer units. After the surfaces of the photosensitive members 1a to 1d are uniformly charged by the chargers 2a to 2d, electrostatic latent images are formed on the photosensitive members 1a to 1d by exposure processes made by the exposure units 3a to 3d according to image signals. After that, the electrostatic latent images are developed by the developers 4a to 4d to form toner images. The toner images on the four photosensitive members 1a to 1d are multiple-transferred onto the intermediate transfer belt 51 by the primary transfer units 53a to 53d, and are further transferred onto a print material P by the secondary transfer units 56 and 57. Transfer residual toners which remain on the photosensitive members 1a to 1d are recovered by the cleaners 6a to 6d, and that which remains on the intermediate transfer belt is recovered by the intermediate transfer belt cleaner 55. The toner images transferred onto the print material P are fixed by a fixing unit 7, thus obtaining a color image.
Conventionally, for the chargers 2a to 2d, it is a common practice to use a corona charging method as a non-contact charging method, which charges by impinging, on the photosensitive member surface, a corona generated by applying a high voltage to a thin corona discharge wire. In recent years, a contact charging method which is advantageous in terms of a low-voltage process, small ozone generation amount, low cost, and the like is prevailing.
FIG. 2 shows a model of the chargers 2a to 2d. An alternating voltage output circuit 28 outputs an alternating output voltage Vac, and a direct-current voltage output circuit 29 outputs a direct-current output voltage Vdc. A voltage charged on the photosensitive member surface by a voltage obtained by superposing the alternating output voltage Vac and direct-current (DC) output voltage Vdc is Vd. In this method, a roller charging member (to be referred to as “charging roller” hereinafter) is brought into contact with the photosensitive member surface, and a voltage is applied to this charging roller to charge the photosensitive member. A voltage applied to the charging roller may be purely a direct-current voltage. However, by superposing an alternating-current (AC) voltage (hereinafter an alternating voltage) on a direct-current voltage to alternately cause discharge processes to the plus and minus sides, a charging process can be uniformly done. As is experimentally confirmed, the relationship among the alternating voltage Vac, direct-current voltage Vdc, and photosensitive member surface potential Vd is as shown in FIG. 3.
That is, by gradually raising the amplitude of the alternating voltage Vac, the photosensitive member surface potential Vd increases accordingly. When the alternating voltage Vac is less than or equal to a predetermined voltage Vac_s, the amplitude of the alternating voltage is nearly proportional to the photosensitive member surface potential. When the alternating voltage Vac is greater than or equal to the predetermined voltage Vac_s, the photosensitive member surface potential Vd matches the direct-current voltage Vdc. Note that Vac represents peak voltage values of the alternating voltage. FIG. 4 shows an electric model of a contact between the charging roller and photosensitive member. As a result of rotation, a contact surface between the charging roller and photosensitive member can be modeled by a capacitive load and resistance connected in series with each other (FIG. 4). It is considered that a discharge phenomenon between the charging roller and photosensitive member contributes to the result shown in FIG. 3. However, in terms of an electric circuit model of the kind shown in FIG. 4, it is considered that increasing the alternating voltage Vac has the effect of lowering an impedance between the charging roller and photosensitive member.
When alternating voltage applied to the charging roller is in the form of a sine wave, a current supplied to the charging roller depends on a capacitive load between the charging roller and photosensitive member and an impedance based on a resistance that changes under the influence of the alternating voltage Vac. FIG. 5 is a graph showing the characteristics of a direct-current Idc that flows through the charging roller when the alternating voltage Vac is applied to the charging roller. By gradually raising the amplitude of the alternating voltage Vac, the direct-current Idc decreases accordingly. When the alternating voltage Vac is less than or equal to a predetermined voltage Vac_s, the amplitude of the alternating voltage is nearly proportional to the direct current. This is because the direct-current voltage Vdc applied to the charging roller and the potential Vd of the photosensitive member have a potential difference, and a charge current Idc corresponding to the potential difference and a load impedance 40 is supplied. In order to stably apply a voltage, which is applied to the charging roller, and also to the photosensitive member while the charging roller contacts the photosensitive member, an amplitude of the alternating voltage should be sufficient to lower the load impedance 40 so as to sufficiently charge a capacitance component of the charging roller/photosensensitive body until Vd=Vdc.
As can be seen from the above description, when the alternating voltage Vac is greater than or equal to a saturation value Vac_s, beyond which there is no increase in direct current Idc in FIG. 5 even if the alternating voltage increases, the photosensitive member surface potential Vd matches the direct-current voltage Vdc. However, as is known, when the amplitude of the alternating voltage Vac is increased, a degradation of the photosensitive member tends to cocur, and at least in a high-temperature, high-humidity environment an abnormal image due to a discharge product tends to be generated. In order to obtain stable charging and to solve the aforementioned problems, a photosensitive member stable potential (Vd=Vdc) has to be obtained by applying a minimum required alternating voltage Vac. However, in practice, the relationship between the alternating voltage Vac applied to the photosensitive member and the direct current Idc is not constant, and changes depending on the film thicknesses of a photosensitive member layer and dielectric layer of the photosensitive member, environmental variations of a charging member and air, and the like. In a low-temperature, low-humidity environment, since the materials of the charging roller are dried, and a resistance increases, the alternating voltage Vac greater than or equal to a given value is required to attain uniform charging. However, even at a lowest voltage value that can obtain charging uniformity in this low-temperature, low-humidity environment, when a charging operation is made in a high-temperature, high-humidity environment, the materials of the charging roller absorb moisture, and the resistance lowers conversely. For this reason, the charging member receives an excessive alternating voltage Vac.
As a result, when the alternating voltage Vac increases, problems of generation of image errors, occurrence of toner fusion, shaving and short lifetime of the photosensitive member due to degradation of the photosensitive member surface, and the like occur. Troubles caused by impedance change characteristics due to the alternating voltage Vac occur due to other factors other than the aforementioned environmental variations. For example, as has already been revealed, the aforementioned troubles are also caused by resistance variations due to manufacturing variations and contaminations of the charging member, capacitance variations of the photosensitive member due to lasting, characteristic variations of a high-voltage generation device in an image forming apparatus, and the like. In order to suppress adverse effects due to excess or deficiency of the alternating voltage Vac, a method of deriving Vac_s is disclosed by Japanese Patent Laid-Open Nos. 2006-276054, 2007-199094, and 2006-267739. Japanese Patent Laid-Open Nos. 2006-276054 and 2007-199094 have proposed a method of deriving Vac_s by calculating Vac-Idc characteristics at the time of unsaturation by measuring Idc using a plurality of Vac values in an Idc unsaturation region, and measuring a saturated current Idc in a saturation region. Also, Japanese Patent Laid-Open No. 2006-267739 has proposed a method of deciding Vac by deriving Vac_s by sweeping Vac from a small value to a large value while detecting Idc.
However, these conventional methods suffer the following problems.
(1) Derivation of the Vac-Idc characteristics by means of plural-point measurements requires a voltage higher than the alternating application voltage Vac used in an actual image forming sequence. This will be described using FIG. 5. Since Vac_s and a saturated current Idc_s as change points of the characteristics vary due to various variation factors, a predetermined Vac value has to be instructed and Idc corresponding to that value has to be detected, so as to derive the characteristics. In order to derive the characteristics shown in FIG. 5, a minimum requirement is to derive primary characteristics from Vac and Idc data at least at two points A and B shown in FIG. 5 using a voltage smaller than Vac_s. Also, Idc of data C at least at one point shown in FIG. 5 using a voltage larger than Vac_s is required. Vac_s can be derived from a straight line derived from A and B and a current value Idc_s at the point C. However, the characteristics based on a value larger than Vac_s are detected using a voltage about 1.5 times of a voltage used in a charging operation since a voltage higher than Vac_s and a sufficiently stable value are required in every environment. A power supply which can sufficiently supply a current at this voltage inevitably requires an increase in size of a high-voltage power supply.
(2) Derivation of a change in Idc by sweeping Vac requires a memory and judgment algorithm since Idc change records have to be derived.
(3) As exemplified in (1), derivation of the characteristics requires much time since an unknown change point Vac_s and known magnitude Idc_s have to be searched.